The production of substituted polysaccharides such as starch and cellulose acetates and carboxymethylated starches and celluloses have typically been produced using batch reactors. During such production methods, the starch or cellulose is suspended in a mixture comprising the reactants and usually a catalyst. Undesirably, such processes require significant amounts of solvents and may require significantly more the stoichiometric amounts or reactants each of which results in waste and the higher associated costs (e.g., financial, environmental, and health) in both the production of the desired product and the treatment and disposal of waste.
For example, cellulose esters are cellulose derivatives prepared by substituting the hydroxyl groups on the glucose units in the cellulose polymer chain by an ester group. Cellulose acetate is likely produced in higher quantities than any other cellulose ester. The conventional process for producing cellulose acetate consists of a pre-treatment step in which the cellulose is treated with glacial acetic acid (seven times the weight of cellulose) for up to four hours. The pre-treated cellulose and acetic acid mixture is then reacted with chilled acetic anhydride (three times the weight of cellulose) in the'presence of a sulfuric acid catalyst. Chilled acetic anhydride is used since the reaction is highly exothermic and involves the release of a considerable amount of heat. The temperature during the initial phase of the reaction is controlled but it may be allowed to increase as the reaction proceeds. The reaction is allowed to continue until all the hydroxyl groups have reacted with acetic anhydride to give a degree of substitution (DS) of 3.R—(OH)3+3(CH3CO)2O→R(OOCCH3)3+3CH3COOHThis product also is called cellulose triacetate (CTA). The complete dissolution of the cellulose indicates the completion of the reaction. The reaction is then terminated by adding water to destroy excess acetic anhydride. The CTA is then hydrolyzed in the presence of water and additional sulfuric acid. Hydrolysis is done for slight deactelyation to lower the DS and make the product acetone soluble. The product is then precipitated by adding excess water and then washed thoroughly to remove all the acetic anhydride and catalyst.
In recent times, researchers have found iodine to be a very effective catalyst for acetylation of alcohols in solvent-less conditions. Researchers then used iodine for the acetylation of cellulose and starch, which are forms of polyalcohol compounds. It was found that iodine could be successfully used as a catalyst for acetylation of cellulose in a short time. However, a solvent is still needed for batch-type processes for reacting cellulose with acetic anhydride, using iodine as a catalyst.
Starch acetate is similarly prepared by reacting starch with acetic anhydride, wherein the hydroxyl groups on the starch are replaced by acetate groups to yield starch acetate. Starch acetate has established thermo-plastic properties. It can be used in bio-degradable plastic applications like packing foams, insulations and disposable plastic goods. However, to date, the cost of manufacturing starch acetate has precluded its commercialization.
Another type of substituted polysaccharide of substantial commercial interest is polysaccharide ethers such as the sodium salt of the carboxymethyl ether of starch, also known as carboxymethyl starch (CMS), which was first prepared in 1924. Since then, carboxymethyl derivatives have been prepared from starches such as corn, amaranth, high amylose corn, potato, wheat, rice, mungbean, Chinese yam, Leucaena glauca seed gum, and wastes therefrom such as corn waste and potato flour waste.
CMS is used very widely in the textile industry as a sizing agent and as an environment friendly, textile printing and finishing agent. CMS is used as a thickening agent in foods, and for personal care and surfactant applications. Partially cross linked carboxymethyl starch, also called sodium starch glycolate, is used as a disintegrant in the pharmaceutical industry. Carboxymethyl starches have also been used in food extrusions as an additive and as an extrusion aid. Recently, researchers have tried using cross-linked carboxymethylated starch for removing heavy metal ions from water. Still further, it also finds applications in medical poultices, adhesives, absorbents, paper making and oil drilling applications.
Typically, CMS is prepared by the reaction of sodium monochloroacetate (SMCA) or chloroacetic acid with starch, in the presence of sodium hydroxide (NaOH) as a catalyst. This reaction is called the Williamson's ether synthesis. In the presence of an aqueous solution of NaOH, starch granules undergo swelling, which increases access to the starch's hydroxyl groups. For the reaction between starch and SMCA to take place, an alkaline medium is required.RONa+ClCH2COONa→ROCH2COONa+NaClNaOH also reacts with SMCA to form sodium glycolate.NaOH+CICH2COONa→HOCH2COONa+NaCl
As mentioned above, a batch reactor is usually used for conducting the foregoing reaction with an aqueous alcohol used as the solvent. An aqueous solvent is used during carboxymethylation of starch, because CMS becomes soluble in water at a DS of 0.1. Sodium hydroxide is added to the starch slurry in aqueous alcohol in the batch reactor along with the reactant SMCA. Known factors for affecting the reaction include the solvent to starch ratio, the type of solvent, the concentration of solvent, the concentration of NaOH, the concentration of SMCA, the temperature, and the duration. For example, increasing temperature tends to increase the reaction rate but if the temperature is too high the starch will gelatinize, which for many applications is not the desired end product and it can cause processing problems. To avoid gelatinization batch reaction temperatures are typically controlled so as to be no greater than about 70° C., which causes the reaction kinetics to be relatively slow.
As mentioned above, batch processing of carboxymethylated starch also involves several solvent-related downsides. For example, a relatively large amount of solvent is needed and it must be heated which takes a relatively long time and is energy intensive. Also, the use of a solvent requires distillation for recycling, which requires further energy and equipment.
Another downside carboxymethylation using a batch process is that it has a relatively low reactive efficiency (RE). For example, a RE of 0.3 was reported for a CMS having a DS of 0.51 formed with a reaction time of 120 minutes, a RE of 0.1 was reported for a CMS having a DS of 0.17 formed with a reaction time of 90 minutes, a RE of 0.45 was reported for a CMS having a CMS of DS 0.49 formed with a reaction time of 100 minutes, and a RE of 0.86 was reported for a CMS having a DS 1.71 formed with a reaction time of 7 hours.
Because of these shortcomings, there has been extensive research on alternative methods of producing starch derivatives. For example, static mixers having been used to prepare hydroxypropyl ethers of starch pastes, but this resulted in gelatinization of the starch with a complete loss of its granular structure. Another method involved using a stirred vibrating fluidized bed to produce hydroxyethyl ethers of potato starch. This method formed starch ethers without losing the starch granular structure, but it was essentially a solid-gas reaction system. Another method involved blending starch with NaOH and SMCA and storing the blend in a dry state at room temperature for a period of weeks or heating it while in a blender for a period of hours, which is still generally considered to be too long to be commercially applicable.
In view of the foregoing, a need still exists for a process to produce substituted polysaccharides such as carboxymethylated and acetylated starches and celluloses having one or more of the following benefits: lower energy requirements, reduced or eliminated the need for solvent, greater reaching efficiency, higher degrees of substitution, higher degrees of polymerization, maintaining a significant degree of the crystallinity, shorter reaction times, reduced waste, reduced costs, and convenient and continuous processing.